Final answer:
Precambrian rocks, such as Vishnu Schist, form the walls of the Grand Canyon and the metamorphic gneiss and schist dominate the Black Canyon's walls. These rocks underwent intense heat and pressure, contributing to their complex structure and appearance.
Step-by-step explanation:
The name given to Precambrian rocks that form the walls of canyons such as the Black Canyon of the Gunnison and the walls of the Grand Canyon is "Vishnu Schist". These rocks are some of the oldest on Earth, nearly 2 billion years old, and they predominantly consist of highly metamorphosed sedimentary rocks. The Vishnu Schist is often associated with Zoroaster granite, which are igneous rocks that intrude into the schists.
In the Black Canyon, the metamorphic rock that dominates the walls is called gneiss, which is blended with schist. Gneiss is known for its intense folding and banding with alternating light and dark layers, showing evidence of having been subjected to intense heat and pressure while buried deep below the Earth’s surface. The schist typically has flat or elongated minerals that align due to the directional pressure they experienced during metamorphism.
In the Grand Canyon, the Vishnu Schist along with Zoroaster granite form the oldest rock foundation. Over time, these rocks were overlaid by younger sedimentary rock layers as the geological history of the area progressed. The Grand Canyon's entire stratigraphic section spans from the Early Proterozoic to the Holocene epoch, with the Vishnu Schist forming the base of this impressive geological record.
The history recorded by these rocks is very complex. The schists and granites within the Grand Canyon and Black Canyon were formed during ancient mountain-building events and were later exposed by erosion. In the Grand Canyon, a well-known feature called the Great Unconformity illustrates a significant gap in the geological record, where these ancient Precambrian rocks lie directly beneath younger Paleozoic sedimentary rock layers.